Detector, reproduction system, receiver and method

ABSTRACT

A detector ( 30 ) is described for detecting a digital signal (B i   o ) out of an input information signal (A i ) which represents a runlength limited sequence, the runlength having a minimal value m. The detector ( 30 ) generates ( 60, 62 ) a preliminary binary signal (B i ) out of the input information signal (A i ). A composed sequence of subsequent bits is identified ( 86 ) within the preliminary binary signal (B i ) which subsequently includes at least a first neighboring bit of a run of length greater or equal than m+1, one or more further runs of length m and at least a second neighboring bit of a run of length greater or equal than intl. A set of sequences are generated ( 84, 86 ). These sequences can be obtained from said composed sequence by changing polarities of binary values within the composed sequence without violating the runlength constraint. The set includes the composed sequence obtained from the preliminary binary signal. A path metric (D) is calculated ( 94, 96 ) for two or more sequences of the set, said path metric (D) being the sum of the branch metrics (d) for the path through the trellis corresponding to the sequence of binary values. The sequence from the set which has the highest likelihood of corresponding to the input sequence represented by the input information signal (A i ) is identified ( 86, 98, 100 ) on the basis of the path metric.

The invention pertains to a detector for deriving a digital signal froman input information signal which represents a runlength limitedsequence, the runlength having a minimal value m.

The invention further pertains to a reproduction apparatus, forreproducing digital symbols stored on a medium, comprising the detector

The invention further pertains to a receiver, for reproducing digitalsymbols from a received signal, comprising the detector.

The invention further pertains to a method for deriving a digital signalfrom an input information signal which represents a runlength limitedsequence, the runlength having a minimal value m.

Optical recording methodology relies heavily on the use of runlengthlimited channel codes. Their purpose is a.o. to match the spectralcharacteristics of the user information to those of the opticalrecording channel. RLL codes restrict the number of consecutive likebits in the coded bit-stream, and may be characterized by twoparameters, namely (d,k) with (d+1) being the minimum and (k+1) beingthe maximum allowed number of consecutive equal bits. Examples of suchRLL codes include the EFM-code (d=2, k=10) employed in the CD, and theEFM+-code (d=2, k=10) employed in the DVD.

During the read-out of optically-recorded information, the storedbit-stream undergoes several kinds of distortion, a.o. owing to thelimited resolution of the laser beam, dynamic defocusing of the readhead and disc tilt. In many cases distortion is such that a simplethreshold detector (TD) is not adequate to reproduce the distorted datawith sufficient accuracy. This can be the case, for example, in DVD,where the recording density is significantly higher than in the CD,resulting in pits and lands of small size and thus in increased intersymbol interference (ISI) from adjacent pits and/or lands. In suchsituations more advanced detection techniques are necessary to achieveacceptable bit-error rates.

Several techniques have been proposed for use with RLL codes. Accordingto a first class of techniques a bit sequence is estimated from theinput signal by means of a maximum likelihood sequence detection, e.g.through the use of a Viterbi algorithm. The Viterbi algorithm is usuallytailored to a fixed partial response and the resulting detector is thencalled a partial response maximum likelihood detector (PRML). It isnormally used as a cascade to an equalizer that serves to shape theoverall response of the channel and the equalizer to the fixed partialresponse used. This detector operates in a bit-by-bit fashion, using thereceived samples and its own estimates for them, in order to recover themost probable transmitted bit pattern. The Viterbi algorithm isdescribed in more detail in “Digital Baseband Transmission andRecording”, by Jan W. M. Bergmans, 1996, chapter 7, pages 301-372.

PRML-detection suffers from an operating speed bottleneck, caused by thebit recursive nature of its critical loop, which comprises at least acomparison and a selection operation. As a result of this speedlimitation, application of PRML detection techniques to the highbit-rates of evolving optical recording technologies becomesincreasingly problematic.

According to a second class of techniques a correcting stage is includedfor detecting and correcting runlength violations in a bit-sequenceobtained in a first stage. The first stage may, for example, beimplemented as a simple threshold detector. The second stage is known asrunlength pushback detection (See for example WO 98/27681).

It is a purpose of the invention to provide a detector according to theintroductory paragraph which detects a digital signal from an inputinformation signal with an error-rate approximating that of aPRML-detector, while its computational complexity is reduced.

It is a further purpose of the invention to provide a reproductionsystem comprising such a detector.

It is a further purpose of the invention to provide a receivercomprising such a detector.

It is also a purpose of the invention to provide a method according tothe introductory paragraph which detects a digital signal from an inputinformation signal with an error-rate approximating that of aPRML-detector, while its computational complexity is reduced.

According to the invention a detector is provided for detecting adigital signal out of an input information signal which represents arunlength limited sequence, the runlength having a minimal value m. Thedetector according to the invention comprises

means for generating a preliminary binary signal out of said inputinformation signal,

means for identifying a composed sequence of subsequent bits within thepreliminary binary signal which subsequently comprises at least a firstneighboring bit of a unipolar sequence of length greater or equal thanm+1, one or more further unipolar sequences of length m and at least asecond neighboring bit of a unipolar sequence of length greater or equalthan m+1, a unipolar sequence being defined as a sequence of bits havingthe same binary value, and bordering at both sides at a bit having theopposite binary value,

means for generating a set of sequences which can be obtained from saidcomposed sequence by changing polarities of binary values within saidcomposed sequence without violating the runlength constraint, the setcomprising the composed sequence obtained from the preliminary binarysignal,

means for calculating a path metric for two or more sequences of saidset, said path metric being the sum of the branch metrics for the paththrough the trellis corresponding to said sequence of binary values,

means for identifying the sequence from said set which has the highestlikelihood of correspondence to the input sequence represented by theinput information signal on the basis of the path metric.

In the detector according to the invention the computation of thelikelihood is restricted to unipolar sequences (runs) or sequences ofruns having a minimal runlength.

The invention is based on the insight that on the one hand the runshaving a minimal runlength are most prone to detection errors as theyhave the highest frequency. On the other hand the path through thetrellis that is based on the preliminary binary signal already has arelatively high likelihood of correspondence to the input sequencerepresented by the input information signal. Hence it suffices toconsider only the set of possible sequences comprising the composedsequence obtained from the preliminary binary signal and those sequenceswhich can be derived from said composed sequence with relatively fewvariations. Hence the number of calculations for estimating the optimalpath is significantly reduced in comparison to the recursivecalculations required in a PRML-detector. Hence, in the detector of theinvention the computational power is used efficiently in that arelatively high number of errors is detected with a relatively smallnumber of computations.

A preferred embodiment of the invention according to claim 2 is based onthe observation by the inventors that the boundaries between subsequentunipolar sequences usually do not shift more than one sample withrespect to each other.

A preferred embodiment of the invention according to claim 3 has theadvantage that unlikely modifications of the composed sequence are leftout of consideration. This further reduces the computational complexitywhile maintaining a good detection performance.

A further preferred embodiment is claimed in claim 4. It has beenobserved that often the boundaries between runs in the signal tend toshift in only one direction. In those cases it is therefore sufficientto evaluate only those variations of the composed sequence. This alsoreduces the computational complexity while maintaining a good detectionperformance.

The invention is in particular applicable to a reproduction system. In areproduction system according to the invention for reproducing digitalsymbols stored on a medium, said reproduction system comprising read outmeans which include means for projecting a first optical beam along anoptical axis at a scanning spot at the medium and for generating aninput information signal representing the digital symbols stored on themedium, by measuring an intensity of an optical beam returned from thescanning spot, said reproduction system further comprising movementmeans for causing a relative movement in a movement direction betweenthe scanning spot and the medium, said reproduction system furthercomprising a detector according to claim 1, 2, 3 or 4 for deriving adigital signal from said input information signal.

A preferred embodiment of the reproduction system according to theinvention is described in claim 6. The tilt measurement enables thesignal detector to predict the kind of distortion which occurs in thesignal. This can be used to restrict the possible modifications in thecomposed sequence to those which have the highest likelihood.

The tilt detection means may be implemented in several ways. For exampleby a separate sensor, which measures a deviation of the position of thebeam reflected from the record carrier. Preferably, however, the tiltdetection means derive a tilt signal from the signal read from therecord carrier. In that way a separate sensor is superfluous. Such tiltdetection means are described for example in the earlier filedapplication WO 00/10165-A1. A practical embodiment of such detectionmeans is claimed in claim 7.

The invention is in particular suitable for application with a channelcode which apart from the runlength constraint also has a repeatedminimum transition runlength (RMTR). The latter prescribes the maximumallowed number of concatenated unipolar sequences of minimum length.Such a code is described for example in WO 99/63671-A1. If the channelcode has a low value for RMTR also the hardware requirements for thedetector are modest, and relatively few variations of the composedsequence have to be computed. EFM+ has a RMTR of 15. The channel codes17PP and EFMCC, both designed for DVR, have a RMTR of 6.

The invention also pertains to a receiver for reproducing a sequence ofoutput symbols from a received signal, the receiver comprising adetector according to the invention which is coupled to a demodulatorfor demodulating the received signal.

Preferably the detector is coupled to the demodulator via an equalizer.

The invention also pertains to a method for detecting a digital signalout of an input information signal which represents a runlength limitedsequence, the runlength having a minimal value m. The method accordingto the invention comprises the steps of

generating a binary signal out of said input information signal,

identifying a composed sequence of subsequent bits within the binarysignal which subsequently comprises at least a first neighboring bit ofa unipolar sequence of length greater or equal than m+1, one or morefurther unipolar sequences of length m and at least a second neighboringbit of a unipolar sequence of length greater or equal than m+1, aunipolar sequence being defined as a sequence of bits having the samebinary value, and bordering at both sides at a bit having the oppositebinary value,

generating a set of sequences which can be obtained from said composedsequence by changing polarities of binary values within said composedsequence without violating the runlength constraint, the set comprisingthe composed sequence

calculating a path metric for two or more sequences of said set, saidpath metric being the sum of the branch metrics for the path through thetrellis corresponding to said sequence of binary values,

identifying the sequence from said set which has the highest likelihoodof correspondence to the input sequence represented by the inputinformation signal on the basis of the path metric.

These and other aspects of the invention are described with reference tothe drawing. Therein

FIG. 1 shows a transmission system in which the present invention can beused,

FIG. 2 shows a reproduction system in which the present invention can beused,

FIG. 3 shows the reproduction system of FIG. 2 in more detail,

FIG. 4 shows a view of a portion of the reproduction system of FIG. 3,according to IV in FIG. 3,

FIG. 5 schematically shows a detector according to the invention forderiving a digital signal,

FIG. 6 shows a unit of the detector in more detail,

FIG. 7 shows a flowchart explaining the operation of the unit of FIG. 6,

FIG. 8 shows a portion of the flowchart of FIG. 7 in more detail,

FIG. 9 shows an example of an input signal, and some variables in theprogram corresponding to the flowchart of FIG. 7,

FIG. 10 shows an example of a composed sequence within the input signaland possible variations,

FIG. 11 shows an embodiment for a tilt detector in an implementation ofthe unit shown in FIG. 6.

In the transmission system according to FIG. 1, a digital signal to betransmitted is applied to a first encoder 3 in a transmitter 2. Theoutput of the first encoder 3 is connected via a second encoder 4 to aninput of a modulator 6. The output of the modulator 6 constitutes theoutput of the transmitter 2. The output of the transmitter 2 isconnected via a transmission medium 8 to an input of a receiver 10. Thereceived signal is applied to an input of a demodulator 12. The outputof the demodulator is connected to an input of an equalizer 14. Theoutput of the equalizer 14 is connected to an input of a detector 16.The output of the detector is connected to a first decoder 17. Theoutput of the first decoder 17 on its turn is coupled to a seconddecoder 18. The output of the second decoder 18 constitutes the outputof the receiver 10.

In the first encoder 3, the digital symbols to be transmitted areconverted into redundant symbols by using an error correcting code. Thiscan e.g. be a convolutional code or a block code such as a Reed-Solomoncode. It is also conceivable that a so-called concatenated coding schemeis used.

The second encoder 4 converts the redundant symbols, obtained from thefirst encoder 3, into channel symbols, by means of a channel codesuitable for transmission via the transmission medium 8.

The channel symbols of the second encoder 4 are modulated on a carrierby the modulator 6. Possible modulation methods are e.g. QPSK, QAM orOFDM.

The modulated signal is transmitted via the transmission medium 8 to thereceiver 10. In the receiver 10, the received signal is demodulated bythe demodulator 12. The demodulated signal is filtered by an equalizer14 to eliminate inter symbol interference caused by the limitedbandwidth of the transmission medium. The detector 16 derives thetransmitted channel symbols from the equalized signal at the output ofthe equalizer 14. In the first decoder 17 the transmitted channelsymbols are converted into intermediate symbols. The latter areconverted into output symbols by the second decoder 18. The seconddecoder 18 provides for error detection and correction. The output ofthe second decoder 18 also constitutes the output of the receiver.

In the reproduction system 20 according to FIG. 2, a record carrier,here an optical disc 22 is read out by read unit 26. It is assumed thatthe data written on the optical disc 22 is coded according to the 8-14EFM coding scheme as is used in the Compact disc standard. However, thepresent invention is also applicable to the 8-16 EFM+ coding scheme asadopted in the DVD (Digital Video Disc) standard. The EFM code has aminimum runlength m (the distance between subsequent bits having a samevalue that are separated by a sequence of bits having the inverse value)of 3, and a maximum runlength of 11. The EFM+ code also has a minimumrunlength m of 3 and a maximum runlength of 11. In another embodimentthe disc is a DVR-ROM disc, which is coded according to the EFM-CCcoding scheme, also having a minimum runlength m of 3 and a maximumrunlength of 11. The EFM-CC coding scheme is described in PH-NL 000074.The disc may otherwise be a DVR-RW disc having a coding scheme 17PP witha minimum runlength m of 2 and a maximum runlength of 8. In againanother embodiment the disc is of a magnetic, or an opto-magnetic type.

The output of the read unit 26 is filtered by an equalizer 28 toeliminate undesired inter symbol interference. The output signal of theequalizer 28 is used by the detector 30 to obtain the sequence ofdetected channel symbols. The operation of the detector 30 will bedescribed later in more detail. The detected channel symbols arereceived by a channel decoder 29 which converts the channel symbols intointermediate symbols. The intermediate symbols are converted by an errorcorrector 32 which substantially reduces the bit error rate. The outputof the error corrector constitutes the output of the reproduction system20.

The record carrier need not be disc shaped, but may otherwise be in theform of a card.

An embodiment of a reproduction system 20 according to the invention forreproducing digital symbols stored on a medium 22 (here an optical disc)is described in more detail with reference to FIG. 3. The reproductionsystem 20 comprises read means 26 which include a unit (not shown) forprojecting a first optical beam 24 along an optical axis 34 at ascanning spot 36 at the medium 22 and for generating an inputinformation signal S_(LS) representing the digital symbols stored on themedium 22, by measuring an intensity of an optical beam returned fromthe scanning spot 36. The reproduction system 20 further comprisesmovement means for causing a relative movement in a movement directionbetween the scanning spot 36 and the medium 22. The movement meanscomprises radial movement means 42, 44, 46 and tangential movement means38. The tangential movement means are in the form of a motor 38 whichrotates the disc 22 around an axis 40. The radial movement meanscomprise a slide 42, which is driven by a slide motor 44. The radialmovement means 42, 44, 46 further comprise a radial transducer 46. Theradial movement means 42, 44, 46 are controlled by a radial servo system48 by means of signals S_(SL) and S_(R). In a reproduction system for arecord carrier in the form of a card the movement means may for examplecomprise a first and a second linear motor for moving the card and thescanning spot with respect to each other in two mutually orthogonaldirections. The read means 26 also comprises a focus actuator (notshown) for maintaining a well focussed scanning spot 36. The focusactuator is controlled by servo system 50. The motor 38, and the servosystems 48 and 50 are controlled by microprocessor 52.

The reproduction system 20 comprises a signal processing unit 28, whichincludes an equalizer for generating an input information signal A_(i),for the detection unit 30. The signal processing unit also provides aradial error signal R_(E) and a focus error signal F_(E) for the servosystem 50.

The reproduction system 20 further comprises a detector 30 according tothe invention for deriving a digital signal B $\frac{o}{i}$

from said input information signal A_(i).

As shown exaggerated in FIG. 4 a tangential tilt α may occur between theoptical axis 34 and a normal vector, which is in this case parallel tothe rotation axis 40, of the medium with respect to the movementdirection v.

The reproduction system comprises tilt detection means 31 for generatinga tilt signal which is indicative for the polarity of the tilt α, andmeans for restricting the set of sequences for which the path metric isdetermined on the basis of the tilt signal.

An output of the detector 30 is coupled to an error corrector 32 forsubstantially reducing the bit error rate.

The system shown in FIG. 3 is also suitable for recording an informationsignal Si at the record carrier 22. To that end the system comprises anencoder 25 for encoding the signal by means of a code enabling errorcorrection, e.g. a code according to the CIRC method. Au output of theencoder 25 is coupled to a channel encoder 27, for encoding the outputsignal of the encoder 25 in a channel code, e.g. EFM. The output of thechannel encoder 27 in its turn is coupled to a write strategy generator33 which generates a pulsed write signal for controlling a radiationsource which is used to generate a beam to write the record carrier. Theradiation source may be the same which is used for scanning the recordcarrier 22.

The tilt detection means 31 may be implemented in several ways. Forexample by a separate sensor, which measures a deviation of the positionof the beam reflected from the record carrier. Preferably however, thetilt detection means derive a tilt signal from the signal read from therecord carrier. In that way a separate sensor is superfluous. Such tiltdetection means are described for example in the earlier filedapplication WO 00/10165-A1. In the embodiment shown the tilt detectionmeans 31 form part of the detector 30.

An embodiment of the detector according to the invention is describedwith reference to FIG. 5. The detector 30 according to the inventioncomprises a first circuit 60 coupled to an input terminal 61 forconverting the input signal A_(i), into an intermediary binary signalB_(i) ^(i). The first circuit 60 is for example a threshold detector.The detector 30 comprises a second circuit 62 which corrects theintermediary binary signal B_(i) ^(i) for runlength violations andtherewith generates a preliminary binary signal B_(i). Such a circuit,also called a runlength pushback detector, is for example described inWO98/27681.

The detector 30 according to the invention also comprises a thirdcircuit 64. This circuit 64 identifies composed sequences of subsequentbits within the preliminary binary signal B_(i) which subsequentlycomprise a first neighboring bit of a unipolar sequence of lengthgreater or equal than m+1, one or more further unipolar sequences oflength m and a second neighboring bit of a unipolar sequence of lengthgreater or equal than m+1. A unipolar sequence is defined as a sequenceof bits having the same binary value, and which sequence borders at bothsides at a bit having the opposite binary value.

When the circuit 64 has identified such a composed sequence within thepreliminary binary signal B_(i), the circuit identifies a set ofsequences which can be obtained from said composed sequence by changingpolarities of binary values within said composed sequence withoutviolating the runlength constraint. The set comprises the composedsequence. The circuit 64 calculates a path metric for two or moresequences of said set. Said path metric is the sum of the branch metricsfor the path through the trellis corresponding to said sequence ofbinary values. The circuit 64 identifies the sequence from said setwhich has the highest likelihood of correspondence to the input sequencerepresented by the input information signal A_(i) on the basis of thepath metric.

FIG. 6 shows the third circuit 64 in more detail. The third circuit hasa first input 70 for receiving the preliminary binary signal B_(i) and asecond input 72 for receiving the input information signal A_(i). Thefirst input 70 is coupled to a first register 74 which has a serialoutput coupled to a first input of a multiplexer 76 and a paralleloutput coupled to a current variation register 78. The signal receivedat the input of the first register 74 is delivered to its serial outputon a first in first out basis. The current variation register 78 iscoupled via a first parallel connection to a best variation register 80.The best variation register 80 has a serial output which is coupled to asecond input of the multiplexer 76. The current variation register 78 iscoupled via a second parallel connection to a window multiplexer 82.This multiplexer 82 selects a subsequence of bits from the sequencestored in the current variation register 78. The window multiplexer 82selects a subsequence which is located within a window around an indexk. The third circuit 64 comprises a modifier 84 coupled to the currentvariation register 78. Upon instruction by a microprocessor 86 themodifier 84 generates variations of the composed sequence identifiedwithin the preliminary binary signal B_(i) by changing polarities ofbinary values within said composed sequence without violating therunlength constraint. The third circuit 64 further comprises anamplitude predicting unit 87 which is coupled to the window multiplexer82. The amplitude predicting unit 87 calculates an expected amplitudeÂ_(k) corresponding to each bit in the sequence on the basis of thewindow around said bit which is selected by the window multiplexer 82.The amplitude predicting unit 87 may be implemented in several ways. Ina first embodiment the amplitude predicting unit 87 calculates thepredicted amplitude Â_(k), from a convolution of the sequence of bitsb_(k) selected by the window with an estimated response function{circumflex over (P)}_(l) of the read channel and the equalizer i.e.:${\hat{A}}_{k} = {\sum\limits_{l = {- L}}^{l = L}{b_{k - l}{\hat{P}}_{l}}}$

Otherwise the predicted amplitude Â_(k) for a particular sequence ofbits may be obtained by averaging the amplitude values A_(i) occurringat the input channel and corresponding to each occurrence of saidparticular sequence of bits. Such a method is described in WO00/12872-A1.

The third circuit is provided with a serial in parallel out register 88which has an input coupled to the input 72 an output coupled to theamplitude register 90. The amplitude register 90 is coupled to a furthermultiplexer 92 which selects amplitude A_(k) within the amplituderegister upon instruction by the microprocessor 86. A distancecalculator 94 calculates a distance d_(k) on the basis of the predictedamplitude Â_(k) and the amplitude A_(k) received via the second input72. The distance d_(k) corresponds to a path metric. The distance isaccumulated by accumulator 96 so as to obtain the path metric D, whichis the sum of the branch metrics for the path through the trelliscorresponding to the sequence of binary values currently being stored inthe current variation register 78.

The third circuit comprises a minimum distance register 98 whichcontains a minimum distance Dmin calculated for the composed sequenceand its variations. The minimum distance Dmin is compared with thedistance D by a comparator 100.

The third circuit 64 has a first in first out register 102 for providinga final binary signal B_(i) ^(o) to output 104. The input of thisregister 102 is coupled to the multiplexer 76.

The operation of the device is described with reference to theflowcharts shown in FIGS. 7 and 8.

The flowchart in FIG. 7 shows the program elements P1 to P17. Theseprogram elements have the following function.

P1: The program is started and a number of variables is initialized. Thevalue Imin is initialized to F, indicating that the previous run is nota run of minimal length. The index i for the samples of the input signalis set to 0. The counter j for counting the number of samples within arun of constant polarity is set to 0.

P2: The index i is incremented with 1.

P3: The amplitude A_(i) of the next sample is read into register 88 andthe corresponding value of the preliminary binary signal B_(i) is readinto register 74.

P4: It is verified whether the value B_(i) of the preliminary binarysignal is the start of a new run, i.e. whether the value B_(i) isopposite to the value B_(i−1) corresponding to the previous sampleA_(i−1).

P5: The counter j for counting the number of samples within a run ofconstant polarity is increased by 1.

P6: A variable r for indicating the length of a run is set equal to thecounter j. The counter j is reset to 0.

P7: It is checked whether the length r of the last identified run isequal to the minimum allowed runlength m.

P8: It is checked whether the flag Imin is T indicating whether the lastidentified run was preceded by one or more runs of minimum allowedrunlength m.

P9: The values B_(i−r−1) until B_(i−2) stored in register 74 aretransferred to the FIFO-register 102.

P10: It is checked whether the flag Imin is F.

P11: The flag Imin is set T and a variable t, for indicating the numberof bits in a train is set to minimum allowed runlength m. A train isdefined as a set of mutually consecutive binary values in thepreliminary binary signal B_(i) comprising exclusively runs of minimumallowed runlength m.

P12: The variable t is increased with m.

P13: The flag Imin is set F.

P14: The input values of samples i-t-r-1 to i-r are loaded from serialin parallel out register 88 into the amplitude register 90.

P15: The values of the preliminary binary signal corresponding to thesamples i-t-r-1 to i-r are loaded from the register 74 into the currentvariation register 78.

P16: A sequence of binary values is calculated which results in thelowest distance measure.

P17: The binary values corresponding to samples i-t-r-1 to i-r aretransferred from the best variation register 80 to the FIFO-register102.

Program element P16 is worked out in more detail in FIG. 8. The programelement P16 comprises the following steps.

PP1: A counter M for counting the number of variations is initialized at0.

PP2: The value D for the distance measure corresponding to a variationis initialized at 0. The index k into the current variation register 78and the amplitude register 90 is set to 0.

PP3: A distance d is calculated from the amplitude A_(k) stored into theamplitude register 90 and the amplitude Â_(k) predicted from the valuesin a window around index k in the current variation register 78. Thevalue D of the distance measure is increased with d. The index k isincreased with 1.

PP4: It is checked whether k is equal to t+2.

PP5: It is checked whether M is equal to 0.

PP6: The minimal distance Dmin is set equal to D.

PP7: The content of the current variation register 78 is loaded into thebest variation register 80.

PP8: It is checked whether the distance D calculated by the accumulator96 is smaller than the minimal distance Dmin stored in the register 98.

PP9: The variation number M is incremented with 1.

PP10: It is checked whether the variation number M is equal to the totalnumber of possible variations Mtot.

PP11: The modifier 84 calculates the next variation of the sequence ofbinary values in the current variation register 78.

The operation of the program illustrated in FIGS. 7 and 8 is furtherexplained with reference to FIG. 9. Therein FIG. 9a shows an example ofa preliminary binary signal B_(i). By way of example it is supposed thatthe preliminary binary signal B_(i) has a minimum allowed runlength m=3.After the variables Imin (FIG. 9e), i (FIG. 9b), and j (FIG. 9c) havebeen initialized at the start P1 of the program, the index i isincremented with 1 in P2. Subsequently in P3 the amplitude A_(i) is readinto the serial in parallel out register 88. Likewise the preliminarybinary value B_(i) assigned thereto by the first 60 and the secondcircuit 62 is read into the serial in parallel out register 74. In stepP4 it is determined that a new run has started, as the preliminarybinary value B₁ has a polarity opposite to its predecessor B₀.Consequently the program continues with P6. Therein the length r (shownin FIG. 9d) of the last identified run is set equal to the counter j(FIG. 9b). Subsequently the counter j is reset to 0. In P7 it isdetermined that the length r=0 of the last identified run is not equalto the minimum allowed runlength m. In P8 it is determined that the flagImin is F and the program continues with P9. As the register 74 does notcontain binary values corresponding to the range of indexes i-r-2 to i-2yet, said program element P9 has no effect and the program continueswith P2. In P2 the index i is increased with one and in P3 the nextvalues for A_(i) and B_(i) are read into the respective registers. In P4it is now determined that no new run has started. The program thencontinues with P5 so that the counter j is increased with 1. The programnow repeats the loop P2-P3-P4-P5 until A₇ and the correspondingpreliminary binary value B₇ have been read into their respectiveregisters 88 and 74. It is then determined in step P4 that a new run hasstarted. Consequently the program continues with P6. Therein therunlength r of the last identified run is set to j, which is 6 in thiscase, and the value of j is reset to 0. In P7 it is subsequentlydetermined that the runlength r is not equal to the minimum allowedrunlength m. The program then continues with P8, where it is determinedthat the flag Imin is F. Now in P9 the preliminary binary values B_(i)with i=0 to 5 are transferred from the register 74 to the FIFO register102 via the multiplexer 76. Subsequently the program continues with theloop P2-P3-P4-P5, reading the amplitude values A_(i) and the preliminarybinary values B_(i) with index i=8 until 13. After the values A_(i) andB_(i) with index i=13 are read in the program steps P4-P6-P7-P8-P9 areexecuted again, having the result that the preliminary binary valuesB_(i) with index i=6 to 11 are transferred to the FIFO-register 102. Nowthe loop P2-P3-P4-P5 is followed. After the values A_(i) and B_(i) withindex 16 have been read in it is determined in P4 that a new run isstarted. In P6 the length r of the last identified run is set equal toj, being 3 in this case, and j is reset to 0. Now in P7 it is determinedthat the length r corresponds to the minimum allowed runlength m. In P10it is determined that the flag Imin is F. Next in P11 the flag Imin isset T and the length t, is set equal to m. The program then continueswith the loop P2-P3-P4-P5 until sample 19 has been received. The programthe continues with P4-P6-P7-P10. In P10 it is determined that the flagImin is T, which has the result that P12 is executed. Therein t isincreased with m. The counter t then has the value 6. The program nowcontinues with the loop P2-P3-P4-P5 until sample 24 has been received.Then P4, P6, P7 is executed. In P7 it determined that the length r ofthe last identified run is not equal to m. Hence the program continueswith P8, where it is determined that Imin is T. Then in P13 the flagImin is reset to F. In P14 the amplitude values A, with index i-t-r-1 toi-r are transferred from the serial in parallel out register 88 to theamplitude register 90. In P15 the corresponding preliminary binaryvalues B_(i) are transferred from the register 74 to the currentvariation register 78. This sequence of preliminary binary values B_(i)forms a composed sequence of subsequent bits within the binary signalwhich subsequently comprises a first bit of a unipolar sequence oflength greater or equal than m+1, one or more further unipolar sequencesof length m and a second bit of a unipolar sequence of length greater orequal than m+1. Subsequently in P16 a set of sequences is obtained fromsaid composed sequence by changing polarities of binary values withinsaid composed sequence without violating the runlength constraint. Thisset comprises the composed sequence. In P16 it is also determined whichsequence of this set has the highest likelihood of correspondence to theamplitude values A_(i) transferred to the amplitude register 90. Thissequence is transferred to the best variation register 80. P16 isexplained in more detail below. After completion of P16, P17 isexecuted, wherein the sequence stored in the best variation register 80is transferred to the FIFO 102. The program then continues to read innew samples with the loop P2-P3-P4-P5. Of course several portions of theprogram may be executed in parallel, for example the reading in ofsamples in said loop and the execution of P16.

The execution of P16 is explained with reference to FIG. 8 and FIG. 10.In FIG. 10a the sequence of values B(i), with i=12 until 19 is shown,which in the example above is loaded into the current variation register78. FIGS. 10b to 10 g show 6 variations of this sequence which arepossible without violating the runlength constraint.

The sequence of preliminary binary values B_(i) shown in FIG. 10a formsa composed sequence of subsequent bits within the binary signal whichsubsequently comprises a first bit of a unipolar sequence of length 5,which is greater or equal than m+1, two unipolar sequences of length m(3) and a second bit of a unipolar sequence of length 6, being greateror equal than m+1. In PP1 the variation number M is set to 0. Thisnumber corresponds to the composed sequence read from the preliminarybinary signal B_(i). In PP2 the distance D is initialized to 0. Also anindex k for the amplitude register 90 and the current variation register78 is initialized to 0. Next, in PP3, a distance D, which is related tothe likelihood that the sequence in the current variation register 78corresponds to the sequence of amplitudes A_(i) in the amplituderegister 90 is calculated. The larger the distance D, the smaller thelikelihood of correspondence. The distance measure is for example

D=Σd_(k),

wherein

d _(k) =ABS(Â _(k) −A _(k))(L 1−norm)

wherein Â_(k) is the expected amplitude calculated on the basis of oneor more binary values within a window around index k in the sequence ofbinary values in the current variation register 78. For that purpose thecomposed sequence loaded into the current variation register 78 may beextended at one or more sides by a plurality of binary values derivedfrom the preliminary binary signal B_(i), the plurality being dependenton the size of the window selected by the window multiplexer 82.

Another distance measure is

d _(k)=(Â _(k) −A _(k))²(L 2−norm)

In the loop PP2-PP3-PP4 the distance D corresponding to a particularpath is calculated. In the embodiment shown the amplitude values A_(k)are selected from the amplitude register 90 by means of the multiplexer92. The binary values from which the amplitude Â_(k) is predicted areselected by means of the window multiplexer 82. The window multiplexer82 and the multiplexer 92 are controlled by the microprocessor 86. Thesummation of PP3 is performed by accumulator 96. In PP5 it is determinedwhether the variation number M is 0, i.e. whether the sequence stored inthe current variation register 78 is the original sequence read in fromthe preliminary binary signal B_(i). This is the case the first timethat the test PP5 is performed. Then, in PP6, the minimal distance Dminstored in register 98 is initialized at the distance D, and in PP7 thecomposed sequence is copied from the current variation register 78 tothe best variation register 80. Subsequently in PP9 the variation numberM is incremented with 1. In PP10 it is checked whether the variationnumber M equals the total number Mtot of variations. The total number ofvariations Mtot is dependent on the number Nr of runs of minimal lengthin the composed sequence, according to the relation Mtot=2Nr+3

By way of example, the 7 possible variations (including the originalsequence) for the sequence of bits 12 to 19 are shown in FIG. 10.

In this embodiment the variations are restricted to the composedsequence, and those sequences which can be obtained therefrom withoutchanging the number of unipolar sequences. For example the sequenceshown in FIG. 10h is not included, as it is highly unlikely that thissequence will correspond to the input signal A_(i).

The number of variations may be further reduced on the basis of theoutput signal α of tilt detector 31. In that case the set of sequencesis restricted to the composed sequence, and the sequences which can beobtained by shifting one or more boundaries of the unipolar sequences inthe original sequence in a first direction, or by shifting one or moreof those boundaries in an opposite direction. In the present exampleeither the sequences shown in FIGS. 10a-10 d would be selected or theset including the original sequence (FIG. 10a) and the variations shownin FIGS. 10e to 10 g. The sequences shown in FIGS. 10b-d or in FIGS.10e-g can be obtained by shifting one or more boundaries of the unipolarsequences in the original sequence in a first direction, or by shiftingone or more of those boundaries in an opposite direction.

As the value of M(=1) is less than the value of Mtot, the programcontinues with execution of PP11. This has the effect that the modifier84 generates the next modification in the current variation register 78.The program continues with PP2 and then repeats the loop PP3-PP4 untilthe distance D has been calculated for that variation. As M equals 1 forthis variation the program continues after the test PP5 with the testPP8. In PP8 it is tested whether the distance D calculated for thatvariation is less than the minimal distance Dmin stored in register 98.If this is true, then the minimal distance Dmin is set equal to thedistance D in PP6, and the content of the current variation register 78is copied to the best variation register 80 in PP7. The comparison PP8is performed by comparator 100.

The program continues until all variations have been generated and theminimal distance Dmin is determined.

FIG. 11 shows a tilt detector 31 which can be used in the unit shown inFIG. 6 to generate a signal a indicating the direction of tilt. The tiltdetector 31 comprises a delay line with delay elements 110 a-d, which isconnected to a first input 108 for receiving the final binary signalB_(i) ^(o). The input 108 and the outputs of the delay elements 110 a-dare coupled to a first and a second pattern detector. The first detector112 generates a detection signal S1 in response to the pattern 10000.The second detector 114 generates a detection signal S2 in response tothe pattern 00001. In another embodiment the patterns are 01111 and11110 respectively. In response to the signal S1 the sample and holdregister 116 samples the corresponding amplitude value A_(i) which itreceives via a delay line 120 from the input 122. Likewise the sampleand hold register 118 samples the corresponding amplitude value A_(i)which it receives via a delay line 120 from the input 122 in response tothe signal S2. The values sampled by registers 118 and 116 respectivelyare averaged by averagers 124 and 126 respectively. The output signalsof these averagers 124, 126 are subtracted from each other by subtractor128, so as to generate the signal α.

A first (I) and a second embodiment (II) of the detector of theinvention were compared with a first (i), a second (ii) and a thirddetector (iii) not according to the invention.

The first detector (i) not according to the invention is a thresholddetector. The second detector (ii) not according to the invention is acombination of a threshold detector and a runlength pushback detector.The third detector (iii) not according to the invention is an adaptiveViterbi detector with a 5-tap response. The Viterbi detector is based ona trellis diagram consisting of 12 states.

The second embodiment (II) of the detector according to the invention isthe same as described with reference to FIG. 6. In that embodiment theset which is evaluated is restricted to those sequences which can beobtained by changing the polarity of at most one of the firstneighboring bit and the second neighboring bit, depending on thedirection of tilt which is estimated.

The first embodiment (I) of the detector differs therefrom in that atilt detector 31 is lacking. In that embodiment the set is evaluatedwhich comprises the composed sequence and that the set is restricted tothe composed sequence, such that the composed sequence comprises onefirst neighboring bit and one second neighboring bit, and thosesequences which can be obtained from the composed sequence withoutchanging the number of unipolar sequences.

The functioning of the five detectors was tested by means of testinformation stored at a DVD-ROM disk. The test information was encodedin the form of a channel code having a minimum runlength of 3 and storedat the disk with a laser having a wavelength of 650 nm. The track pitchamounted 0.74 μm and the linear density was 4.6% higher than specifiedin the DVD-standard. To test the robustness of the detectors at varyingdegrees of distortion, the tangential disk tilt was varied between−0.80° and +0.80°. It was determined within which tilt margin the SymbolError Rate (SER) was less or equal than 10⁻⁴. The results are shown inthe following table.

Detector tilt margin (°) improvement i 0.8135 ii 1.0305 iii 1.2980 26.0%I 1.2612 22.4% II 1.3146 27.6%

As expected the Viterbi detector (iii) allows a higher tilt margin(+26.0%) than the runlength pushback detector (ii). Surprisingly thefirst embodiment of the detector according to the invention (I) providesin an improvement of the tilt margin (+22.4%) which is comparable tothat obtained with the Viterbi-detector, while the number of possiblebit sequences which it evaluates is significantly less. Even moresurprising is that the second embodiment of the detector according tothe invention (II) provides in an improvement of the tilt margin (27.6%)which is even higher than that obtained by the Viterbi detector, whilethe number of bit sequences which is evaluated is about half thatevaluated by the detector according to embodiment I of the invention.

It is remarked that the scope of protection of the invention is notrestricted to the embodiments described herein. Neither is the scope ofprotection of the invention restricted by the reference numerals in theclaims. The word ‘comprising’ does not exclude other parts than thosementioned in a claim. The word ‘a(n)’ preceding an element does notexclude a plurality of those elements. Means forming part of theinvention may both be implemented in the form of dedicated hardware orin the form of a programmed general purpose processor. The inventionresides in each new feature or combination of features.

What is claimed is:
 1. Detector (30) for detecting a digital signal(B_(i) ^(o)) out of an input information signal (A_(i)) which representsa runlength limited sequence, the runlength having a minimal value m,the detector (30) comprising, means (60, 62) for generating apreliminary binary signal (B_(i)) out of said input information signal(A_(i)), means (86) for identifying a composed sequence of subsequentbits within the preliminary binary signal (B_(i)) which subsequentlycomprises at least a first neighboring bit of a unipolar sequence oflength greater or equal than m+1, one or more further unipolar sequencesof length m and at least a second neighboring bit of a unipolar sequenceof length greater or equal than m+1, a unipolar sequence being definedas a sequence of bits having the same binary value, and bordering atboth sides at a bit having the opposite binary value, means (84, 86) forgenerating a set of sequences which can be obtained from said composedsequence by changing polarities of binary values within said composedsequence without violating the runlength constraint, the set comprisingthe composed sequence obtained from the preliminary binary signal, means(94, 96) for calculating a path metric (D) for two or more sequences ofsaid set, said path metric (D) being the sum of the branch metrics (d)for the path through the trellis corresponding to said sequence ofbinary values, means (86, 98, 100) for identifying the sequence fromsaid set which has the highest likelihood of correspondence to the inputsequence represented by the input information signal (A_(i)) on thebasis of the path metric.
 2. Detector according to claim 1,characterized in that the composed sequence comprises one firstneighboring bit and one second neighboring bit.
 3. Detector according toclaim 1 or 2, characterized in that the set is restricted to thecomposed sequence (FIG. 10a), and those sequences (FIG. 10b-FIG. 10g)which can be obtained from the composed sequence without changing thenumber of unipolar sequences.
 4. Reproduction system (20) forreproducing digital symbols stored on a medium (22), said reproductionsystem (20) comprising read out means (26) which include means forprojecting a first optical beam (24) along an optical axis (34) at ascanning spot (36) at the medium and for generating an input informationsignal (A_(i)) representing the digital symbols stored on the medium(22), by measuring an intensity of an optical beam returned from thescanning spot (36), said reproduction system (20) further comprisingmovement means (38, 42, 44, 46) for causing a relative movement in amovement direction (v) between the scanning spot (36) and the medium(22), said reproduction system (20) further comprising a detector (30)according to claim 3 for deriving a digital signal (B_(i) ^(o)) fromsaid input information signal (A_(i)).
 5. Reproduction system accordingto claim 4, characterized by tilt detection means (31) for generating atilt signal (cc) which is indicative for the polarity of a deviationbetween the optical axis (34) and a normal vector (40) of the medium(22) with respect to the movement direction (v), and means forrestricting the set of sequences for which the path metric is determinedon the basis of the tilt signal (α).
 6. Reproduction system according toclaim 5, characterized in that the tilt detection means (31) comprisemeans (112, 114) for detecting first symbol patterns of the form xxxxoand second symbol patterns of the form oxxxx, wherein o and xrespectively represent a first and a second polarity, means (124, 126)for measuring the average amplitude level in the input informationsignal (A_(i)) corresponding to the first and to the second bit pattern.7. Reproduction system according to claim 6, characterized by means forrestricting the set to those sequences which can be obtained by changingthe polarity of at most one of the first neighboring bit (FIGS. 10a-d)and the second neighboring bit (FIG. 10a, FIGS. 10e-g), depending on thedirection of tilt which is estimated.
 8. Detector according to claim 1,characterized in that the set is restricted to the composed sequence(FIG. 10a), and the sequences (FIGS. 10b-d; FIGS. 10e-g) which can beobtained by shifting one or more boundaries of the unipolar sequences inthe original sequence in a first direction, or by shifting one or moreof those boundaries in an opposite direction.
 9. Receiver (10) forreproducing a sequence of output symbols from a received signal, thereceiver comprising a detector (16) according to claim 8, which iscoupled to a demodulator (12) for demodulating the received signal. 10.Receiver (10) according to claim 9, characterized in that the detector(16) is coupled to the demodulator (12) via an equalizer (14). 11.Reproduction system (20) for reproducing digital symbols stored on amedium (22), said reproduction system (20) comprising read out means(26) which include means for projecting a first optical beam (24) alongan optical axis (34) at a scanning spot (36) at the medium and forgenerating an input information signal (A_(i)) representing the digitalsymbols stored on the medium (22), by measuring an intensity of anoptical beam returned from the scanning spot (36), said reproductionsystem (20) further comprising movement means (38, 42, 44, 46) forcausing a relative movement in a movement direction (v) between thescanning spot (36) and the medium (22), said reproduction system (20)further comprising a detector (30) according to claim 1 or 2 forderiving a digital signal (B_(i) ^(o)) from said input informationsignal (A_(i)).
 12. Reproduction system according to claim 11,characterized by tilt detection means (31) for generating a tilt signal(α) which is indicative for the polarity of a deviation between theoptical axis (34) and a normal vector (40) of the medium (22) withrespect to the movement direction (v), and means for restricting the setof sequences for which the path metric is determined on the basis of thetilt signal (α).
 13. Reproduction system according to claim 12,characterized in that the tilt detection means (31) comprise means (112,114) for detecting first symbol patterns of the form xxxxo and secondsymbol patterns of the form oxxxx, wherein o and x respectivelyrepresent a first and a second polarity, means (124, 126) for measuringthe average amplitude level in the input information signal (A_(i))corresponding to the first and to the second bit pattern. 14.Reproduction system according to claim 13, characterized by means forrestricting the set to those sequences which can be obtained by changingthe polarity of at most one of the first neighboring bit (FIGS. 10a-d)and the second neighboring bit (FIG. 10a, FIGS. 10e-g), depending on thedirection of tilt which is estimated.
 15. Receiver (10) for reproducinga sequence of output symbols from a received signal, the receivercomprising a detector (16) according to one of the claims 1, 2, or 8which is coupled to a demodulator (12) for demodulating the receivedsignal.
 16. Receiver (10) according to claim 15, characterized in thatthe detector (16) is coupled to the demodulator (12) via an equalizer(14).
 17. Method for detecting a binary signal (B_(i) ^(o)) out of aninput information signal (A_(i)) which represents a runlength limitedsequence, the runlength having a minimal value m, the method comprisingthe steps of generating a preliminary binary signal (B_(i)) out of saidinput information signal (A_(i)), identifying a composed sequence ofsubsequent bits (FIG. 10a) within the binary signal which subsequentlycomprises at least a first neighboring bit of a unipolar sequence oflength greater or equal than m+1, one or more further unipolar sequencesof length m and at least a second neighboring bit of a unipolar sequenceof length greater or equal than m+1, a unipolar sequence being definedas a sequence of bits having the same binary value, and bordering atboth sides at a bit having the opposite binary value (FIG. 7),generating a set of sequences (FIGS. 10a-g) which can be obtained fromsaid composed sequence by changing polarities of binary values withinsaid composed sequence without violating the runlength constraint, theset comprising the composed sequence (PP11) obtained from thepreliminary binary signal, calculating a path metric (PP3) for two ormore sequences of said set, said path metric being the sum of the branchmetrics for the path through the trellis corresponding to said sequenceof binary values, identifying the sequence from said set which has thehighest likelihood of corresponding to the input sequence represented bythe input information signal on the basis of the path metric (PP7).